Optical measurement devices are commonly used for concentration measurements of gaseous substances. A number of different techniques, such as e.g. Differential Optical Absorption Spectroscopy (DOAS) applications, tuneable laser spectroscopy applications and Fourier transform spectroscopy, and corresponding measurement devices utilize radiation absorption by gas molecules for calculation or estimation of a certain molecular concentration along a radiation path, so called in sight absorption spectroscopy. The basic concept typically comprises a radiation source radiating a spectrally known electromagnetic radiation through a gas volume. Each gas molecule species in the volume will absorb the radiated photon energy according to its own unique absorption spectrum, determined by the discreet energy transitions possible in the particular molecule species (primarily dependent on electron, vibrational and rotational energy states for the particular molecule electrons). A spectrometer is typically used for determination of the spectrum of the radiation after possible absorption in the measurement volume. The measured spectrum is compared to the known spectrum of the radiation source and the unique absorption spectra for the gas species along the radiation path are identified. The concentration of a detected gas in the measurement volume is determined by the relative absorption measured for its spectrum according to the Beer-Lambert law. Absorption spectroscopy for gas analysis, as described above, is used in many applications for measurements of air pollution, such as general air pollution, exhaust gases from combustion engines, gaseous emissions from chimneys, volcanoes etc.. Commercially available measurement devices for these purposes usually comprise a telescopic device for transmitting light in a collimated way over a large distance. This can be accomplished by placing a light source in the focal point of a concave mirror or a lens to produce a substantially parallel beam of light. The collection of the light is made in a similar fashion by placing a detector in the focal point of a lens or a concave mirror.
EP 0 472 637 B1 shows a device for absorption measurements capable of both emitting and receiving light. The device is primarily made for measuring air pollution gases and is therefore often mounted on a chimney, a roof top or some other suitable outdoor location. The device has a light source positioned centrally in a tube. A concave mirror collimates the light in a forward direction towards a mirror positioned at a distance of 10 m to 10 km. The mirror redirects the light back into the tube, where another, bigger mirror placed behind the first concave mirror, focuses the light onto a detector faced backwards and positioned in the forward direction compared to the light source. A moveable shielding element is placed between the light source and the detector. During measurement, the shielding element is placed to block direct radiation from the light source to reach the detector. However, the shielding element can be folded away, while at the same time a second shielding element is placed to block the forward exit of the tube, so that the detector will only measure direct radiation from the light source. In that way reference measurements can be made of the light source and absolute concentration measurements of gas species can then be made.
WO90/04761 shows a similar device for absorption measurement, also for measuring air pollution. This device is also mounted in a pipe housing having a light source, a concave mirror at the back collimating the light from the light source forward towards a reflective mirror arranged at a distance from the light source. A second mirror is placed between the light source and the concave mirror with its reflective surface facing towards the concave mirror. A detector is placed between the second mirror and the concave mirror so that it is in the focal point of the concave mirror for light received by the device, while being shaded from the light source by the second mirror.
The above devices for air pollution measurements are delicate optical pieces of equipment placed in a rough outdoor environment. The rough environment implies that the devices fairly often have to be calibrated, aligned and maintained. A mirror might get dirty affecting its reflective spectrum. Aging of the detector, mirror and/or lenses might change the spectral behaviour of the device. Aging of the light source might alter the emitted radiation spectrum. The above mentioned problems will lead to a gradually decreased accuracy of the measurement results, while not necessarily affecting the precision of consecutive measurements. The latter makes the problem hard to discover. To be able to produce not only precise but also accurate measurement results maintenance of the equipment and recalibration of its spectral characteristics is necessary at a regular basis. The location of the measurement devices at for example industrial chimneys or roof tops makes the maintenance difficult, dangerous, time consuming and thus expensive.